The present invention relates to a system for allocating resources in a cellular mobile radio network. The object is to select a radio channel to convey a call between a terminal and a base station of the network. Each cell has a base station. Thus a cell and its base station can be referred to interchangeably, so to speak.
One typical example of a cellular network is the GSM, now in widespread use. To simplify the following description, specific reference will be made to the GSM, but this is not limiting on the scope of the invention.
The field of the invention is therefore that of cellular networks. A cell uses transmission frequencies which are not used in any of the cells which are its near neighbors. Using the standard hexagonal representation of cells, any cell has six near neighbors.
In GSM networks, each cell has a beacon frequency referred to as BCCH and used in particular to set up the initial connection from a terminal to the network, i.e. to convey signaling information required by the terminal as soon as it is switched on.
It is therefore standard practice to employ a re-use pattern of twelve or even more cells for these beacon frequencies. To simplify the description of the invention, a pattern with seven cells is used here: a separate beacon frequency is allocated to each of the cells forming a pattern made up of a central cell and its six near neighbors. The most reliable solution, in terms of network operation, is naturally to use the same pattern of seven cells over all the available frequencies, and in particular over the traffic frequencies used for calls.
However, if the pattern with seven cells is applied to all the frequencies used in the network, the required number of calls in a cell cannot be supported. This is because the number of calls on each frequency is a network constant (with a value of one in FDMA systems or eight in the GSM). Also, the number of frequencies available in a cell falls as the rate of re-use falls. The rate of re-use is defined as the reciprocal of the number of cells in the re-use pattern.
The need to use a pattern with a higher rate of re-use for at least some traffic frequencies has therefore become apparent. A pattern with four cells has been used. A pattern with three cells has also been used, and has the highest rate of re-use in a cellular architecture where the use of the same frequency in two adjacent cells is prohibited. The pattern with three cells is formed by three adjacent hexagons having a common apex.
It follows from the foregoing considerations that the frequencies used in the network can be divided into primary frequencies and secondary frequencies. The primary frequencies, which conform to the pattern of re-use with seven cells, provide the required high quality of service and the secondary frequencies, which conform to a pattern with a higher rate of re-use, for example ⅓, increase the volume of calls.
Any cell therefore has a set of primary frequencies and a set of secondary frequencies and each set comprises at least one frequency. The beacon frequency of a cell naturally belongs to its set of primary frequencies. For convenience, a cell is identified by a primary color and a secondary color which respectively correspond to the set of primary frequencies and to the set of secondary frequencies allocated to it.
When the terminal is in standby mode, i.e. when it is logged onto the network and is awaiting a call, it is connected to a local cell by a standby channel referred to as the SDCCH. When a call involving the terminal is set up, the network must select an available resource in the local cell.
If that resource is selected at random, or in accordance with inappropriate criteria, especially if the network is heavily loaded, it may not support a call of sufficient quality, in particular from the point of view of the terminal. This situation will initiate a change of resource, with the aim of finding a new channel which is better for the terminal concerned. It is naturally preferable to avoid this procedure, which is referred to as xe2x80x9chandoverxe2x80x9d, since it increases signaling within the network, which becomes increasingly undesirable as the load on the network increases. Moreover, if the resource initially selected proves to be of mediocre quality, this is very probably because it is being used for other calls which have already been set up and which are interfering with the call of the terminal in question, which terminal will itself naturally interfere with those other calls. For the same reason, this will in turn lead to the risk of new handovers.
The problem of choosing a resource therefore arises at the time of the initial allocation, i.e. at the time of call set-up, and also at the time of handover. In this latter case it is a matter of finding a new resource.
Thus an object of the present invention is a system for allocating to a terminal a resource whose potential quality is high.
The system of the invention is used in a cellular mobile radio network in which each cell of the network is identified by a primary color and a secondary color which correspond to transmission frequencies allocated to it, namely and respectively a set of primary frequencies and a set of secondary frequencies whose rate of re-use is higher than that of the primary frequencies, and the terminal is connected to a local cell. The system knows a potential level of interference received by said terminal and allocates said resource to the terminal on a primary frequency or on a secondary frequency according to whether said potential level of interference is respectively above or below a first distribution threshold.
The environment of the terminal is therefore taken into account in selecting the resource. This avoids choosing a frequency that the terminal receives at a relatively high level.
The system advantageously knows a cumulative probability of said potential level of interference and fixes the first distribution threshold by comparing said cumulative probability with the ratio of the number of resources of the set of primary frequencies to the total number of resources.
This optimizes the first distribution threshold allowing for the frequencies available in the cell.
The set of primary frequencies of the local cell includes a beacon frequency and at least one traffic frequency and the system allocates said resource to the terminal on said traffic frequency or on the beacon frequency according to whether the potential level of interference is respectively above or below a second distribution threshold.
This is an improvement to the basic principle of the invention.
The system therefore preferably knows a cumulative probability of said potential level of interference and preferably fixes the second distribution threshold by comparing said cumulative probability with the ratio of the number of traffic frequency resources of the set of primary frequencies of the local cell to the total number of resources.
In a first embodiment of the system the terminal measures the levels received from a particular number of adjoining cells which have a different primary color to the local cell and the system knows the number of adjoining cells identified as having the same secondary color as the local cell and establishes the potential level of interference by means of an increasing function of that number.
In a second embodiment of the system, the terminal measures the levels received from a particular number of adjoining cells which have a different primary color to that of the local cell, and the system produces the potential level of interference by summing the levels received from adjoining cells with the same secondary color as the local cell.
In this case, if the system also knows the levels received by the terminal from surrounding cells identified as using secondary frequencies adjacent those of the local cell, the network being designed so that a received adjacent frequency is allocated a predetermined attenuation coefficient, the potential level of interference is increased by the product of that attenuation coefficient and the sum of the levels received from said surrounding cells.
This refines the estimated level of interference.
The potential level of interference is advantageously weighted by the level received by the terminal from the local cell.
If the network has a separate beacon frequency in each set of primary frequencies the level received from one of the adjoining cells is preferably measured on the beacon frequency allocated to it.
The invention is highly beneficial for initial allocation of the resource when a call is set up with the terminal.
The invention is of great benefit for initially allocating the resource on setting up a call with the terminal.